In many medical situations it is desirable to quantitatively determine, or measure, the recirculation rate or the flow rate of a biological or medical fluid to increase the benefits of a therapeutic treatment, or alternatively, to decrease the time required for the treatment. This information is also useful for diagnostic purposes. For example, hemodialysis (herein "dialysis") is an uncomfortable medical procedure. It is, therefore, widely recognized as desirable for therapy to minimize the amount of time required to complete the dialysis procedure while still achieving a desired level of treatment.
In dialysis, a joint is typically surgically created between a vein and artery of a patient. This joint provides a blood access site where a blood loop, comprising an inlet or arterial line to a dialysis apparatus and an outlet or venous line from the dialysis apparatus, may be connected to the patient. The patient's blood flows through the joint from the artery to the vein. The inlet line draws blood to be treated from the patient through a first cannula inserted into the joint, while the outlet line returns treated blood to the patient through a first cannula inserted into the joint, while the outlet line returns treated blood (i.e., after dialysis), to the patient through a second cannula inserted into the joint between the first cannula and the vein. The joint may be an arteriovenous fistula, which is a direct connection from one of the patient's arteries to one of the patient's veins. Alternatively, the joint may be a synthetic or animal organ graft connecting the artery to the vein. As used herein, the term "fistula" refers to any surgically created or implanted joint between one of the patient's veins and one of the patient's arteries, however created. More generally, the terms "shunt" or "access" refer to any similar joint, either in a hemodialysis patient or in another area.
A portion of the treated blood, after being returned to the patient by the outlet line, may recirculate within the fistula and commingle with untreated blood being withdrawn from the patient by the inlet line. The result is a decrease in the efficiency of the therapy, as some of the volume of blood in the inlet line has already been treated. The inefficiency in turn requires a longer treatment period with negative effects upon the patient. This recirculation and the resulting commingling is dependent in part on the rate at which the blood is withdrawn from and returned to the patient. In order to select the most efficient flow rate and thus achieve the quickest possible treatment time, it is desirable to know the proportion of recirculated treated blood in the blood being withdrawn from the patient by the inlet line.
A method and apparatus for quantitatively determining the degree of recirculation in a fistula is described in U.S. Pat. No. 5,510,717, incorporated herein by reference in its entirety. In the method disclosed in the patent, a bolus of marker fluid having a conductivity different from that of blood is injected through an injection site in the outlet line. A differential conductivity monitor quantitatively measures the degree of recirculation in a fistula by comparing the conductivity of blood entering the fistula to the conductivity of blood being withdrawn from the fistula. This measurement is made as blood flows in the predominant direction from artery to vein. Conductivity is not the only property used in the prior art for sensing recirculated blood. Among other fluid properties utilized for the measurement of recirculated blood are temperature of the fluid and speed of conduction of sound. Some of these measurement techniques also involve injection of a bolus of marker material into the blood flow. For all the above methods of detecting recirculating blood, the techniques for handling the blood loop remain essentially the same.
Measurement of the fistula's flow rate is also desirable. In the patient, the fistula gradually loses its ability to efficiently transport blood from artery to vein. Fat and other deposits build up within the fistula and flow is gradually reduced. Eventually, the fistula must be replaced. This repetitious replacement can account for half the long term cost of dialysis treatment. In the interest of making these replacements as infrequent as possible, it is necessary to know whether the fistula flow rate is adequate. It can be readily appreciated that ascertaining the flow rate through the fistula is very important to long term care of the patient.
The flow rate through the fistula or the access flow rate ("Q-access") can be determined utilizing the same equipment and procedures used for monitoring of the recirculation rate, but with a reversed flow of blood to the fistula. To measure Q-access the inlet line is disconnected from the first line and connected to the second cannula while the outlet line is disconnected from the second cannula and connected to the first cannula. Blood is withdrawn from the fistula at a location downstream of the location at which it is returned to the fistula. By injecting a bolus of a marker fluid having a property such as conductivity different from that of blood, and measuring and comparing the values of that property in the inlet and outlet lines, the Q-access or access flow rate of the dialysis apparatus may be determined.
This reversal of the blood flow is accomplished by manually switching the tubing and cannula which connect the dialysis apparatus to the patient, or switching the lines at a point removed from the patient. This switching of the lines is well known in the prior art, regardless of the equipment or exact blood property being used to measure the recirculation rate and access flow rate.
Even in situations where it is desired to measure Q-access without measuring recirculation flow, this manual switching is required as the desirable flow of blood into and out of the fistula for dialysis treatment is the opposite of that required for measuring Q-access.
The switch of tubing and cannulae is a source of danger to the patient and to health care personnel. Normally, the dialysis machine, fistula, and associated tubing provide a closed loop or closed aseptic system. Switching of the tubing presents risks that aseptic techniques may be compromised by the operator. Ultrasonic air bubble protection may be lost. Blood may be sprayed or spilled onto the operator, raising the risk of blood borne infection. Finally, when dealing with open blood lines, there is a chance of exsanguination of the patient.
It is against this background that the significant advances of the present invention developed.